This invention relates to the production of monocrystalline materials, and more particularly, to a process for producing an epitaxially grown continuous ribbon of monocrystalline composition.
There is a need for a reliable and economical method of continuously producing high-quality single-crystal (monocrystalline) materials such as semiconductors. Such a method has application to devices such as solar cells, transistors, displays, duplicating equipment, photodetectors, and optoelectronic devices.
At the present time, single-crystal materials are commonly prepared by:
1. growing single-crystal ingots; PA1 2. sawing the ingots into thin slices; PA1 3. polishing the slices to yield single-crystal surfaces; PA1 4. epitaxially growing single-crystal layers on the slices; PA1 5. processing and shaping devices on the epitaxial layers. PA1 1. ingot growth is a high-temperature method which also produces defects and consumes energy; PA1 2. sawing is a slow process requiring expensive machinery and is wasteful of single-crystal material; PA1 3. polishing is wasteful of single-crystal material; PA1 4. the series of steps requires batch processing and precludes continuous processing.
The series of steps has the following disadvantages:
There have been various attempts to create continuous processes by pulling dendritic crystals from melts or by fusing polycrystalline ribbons of silicon. For example, refer to the following U.S. Pat. Nos.: Spenke et al 3,341,376; Shaikh 3,441,454; Sirtl et al 3,900,943; Kendall et al 3,969,746 and Lesk 4,027,053.
These approaches do not allow one arbitrarily to choose crystallographic orientations which provide optimum device performances. They also yield polycrystalline material or unacceptably high concentrations of crystalline defects in the resulting ribbons.
I have invented a method by which sufficiently thin strips of single-crystal material can be bent and shaped into a continuous belt without degrading the desired high degree of quality characteristic of monocrystalline material. In addition, the crystallographic orientation of the entire major surface of the ribbon can be chosen at will to yield optimum device performance.
Briefly, my method comprehends the use of a continuous single-crystal belt which provides a substrate for continuous epitaxial growth of a variety of devices by methods such as vapor-phase epitaxy (VPE) and liquid-phase epitaxy (LPE). The epitaxial device layers are separated in continuous fashion from the belt, allowing the belt to be re-used.
Because such epitaxial growth is at relatively low temperatures and is used to grow only the quantity of material necessary for device fabrication, my new continuous process is fundamentally less wasteful of energy and materials and is more economical and reliable than the prior art.
In addition, processing according to my invention allows epitaxial growth on generally available substrate materials whose effective lattice parameter is slightly and controllably adjusted by the degree of bending.
Accordingly, it is an object of the present invention to provide an effective, advantageous new method of producing high-quality single-crystal materials such as semiconductors, and particularly a process for epitaxial growth of such monocrystalline materials for producing indefinite lengths of epitaxially grown ribbon of various materials, including metals, compounds or alloys of various inorganic elements as well as other material capable of single-crystal orientation.
Among the various other and more specific objects of the invention may be noted the provision of such a process which allows extremely high-quality, substantially defect-free monocrystalline ribbon to be continuously grown in any deisred length, or to be of indefinite length; which allows the preselection of crystalline orientations of the ribbon; which substantially minimizes or avoids wastage of energy and materials; which achieves reliable, uniform, repeatable, continuous and consistent results; and which can be carried out indefinitely without need for batching, reloading, reconfiguring or the like.
A further object is the provision of a method or process for forming an endless belt of monocrystalline material on which the epitaxial ribbon is grown, wherein the belt can not only be formed substantially without discontinuities or imperfections and in desired crystallographic orientation but also can be reused time and time again.
Generally, my new process for growing an epitaxial ribbon of monocrystalline material in accordance with the invention comprises forming an endless belt of monocrystalline composition and driving the belt about a closed path to bring portions thereof sequentially to epitaxial growth and ribbon stripping zones. There is epitaxially grown on portions of the belt in the epitaxial growth zone at least one epitaxial layer. The process further involves stripping such epitaxial layer(s) in the stripping zone to form an epitaxial ribbon of indefinite length. Finally, the ribbon is wound upon a mandrel for storage or transport before further processing.
The endless belt of monocrystalline composition is formed, according to my new invention, by slicing a boule of the first monocrystalline composition into flat strips of uniform thickness. Ends of the strips are then beveled to provide beveled end surfaces each of preselected crystallographic orientation. A plurality of the strips are joined in end-to-end relationship to define a bevel-edged notch between the strips. Then, material of said monocrystalline composition is epitaxially grown on beveled end surfaces to fill the notches. Excess thickness of the material grown into the notches is polished away to a condition of flatness coincident with surfaces of the strips, providing an elongated double-edged belt of uniform thickness having beveled end surfaces. The opposite ends of the belt are bent back upon each other in end-to-end relationship to define a final bevel-edged notch between the ends of said strip, such bending being at a radius not less than a critical minimum value. This last notch is again filled by further epitaxial growth on its beveled end surfaces, and excess thickness of this last growth is polished to a condition of flatness coincident with adjacent surfaces of those strips which defined said final notch, thus providing the endless belt.
Other objects and features of my invention are apparent or are pointed out hereinbelow.